1. Field of the Invention
The present invention relates to lubricant compositions and, more specifically, to methods and means of increasing the resistance of such compositions to oxidative degradation. The lubricant compositions of the invention may be particularly useful as hydraulic oils, gear oils, and compressor oils, particularly where aromatic compounds are of concern due to toxicity.
2. Background of the Art
In recent years the lubricants industry has made increasing use of solvent-refined and hydroprocessed hydrocarbon base oils. This is because traditional hydrocarbon, i.e., mineral oils (classified as Group I base oils by the American Petroleum Institute (API)) that have been used for many decades generally contain a high aromatic content, typically in the vicinity of 20 percent by weight (wt %). The high aromatic content tends to reduce the base oil quality by reducing its viscosity index and may also present toxicity issues, making these oils undesirable for some applications.
In view of the potential problems with aromatic content, the lubricants industry is currently moving toward greater use of Groups II and III base oils, which are base oils that achieve substantially reduced aromatic contents through hydro-processing methodologies. These base oils generally contain less than 10 wt % of aromatics and less than 300 parts per million (ppm) of sulfur. This level may then be reduced even further, to about 1 wt % aromatics and almost undetectable amounts of sulfur and nitrogen, by subjecting the Groups II and III base oils to post-treatment processes. Such processes may include hydrocracking and iso-dewaxing. Over the course of the next decade use of Group II and III oils is expected to grow rapidly as the industry transitions to these higher quality base oils.
Unfortunately, most lubricant applications require a certain minimum level of resistance to oxidative degradation. To achieve desirable levels, antioxidant additives may be used, and/or supplemental oils, containing naturally occurring antioxidants, may be included in the compositions. In most cases, antioxidants based on aromatic structures, such as aromatic amines and/or aromatic phenolic derivatives, are preferred for reasons of performance, cost, convenience, or a combination thereof. However, adding aromatic amines to Group II or III base oils vitiates, to at least some extent, the benefits derived from selecting a Group II or III base oil, instead of a Group I base oil, in the first place.
Because of the frequent conflict between achieving appropriate resistance to oxidative degradation and avoiding or reducing aromatic content, researchers in the lubricants field have continued to seek out new lubricant additives and/or formulations that offer thermoxidative stability without increased aromatic contents.